Thermophilic fungi have been isolated from almost any soil, even in the temperate zones, prompting the remark "the ubiquitous distribution of organisms, whose minimal temperature for growth exceeds the temperature obtainable in the natural environment from whence they were isolated, still stands as a 'perfect crime' story in the library of biological systems" (Tendler et al., 1967). Whether their presence in soil is because of their growth therein or a consequence of dissemination of their spores from compost heaps that occur world-wide has not been easy to resolve since the opaqueness of soil precludes microscopic examination of fungal growth. Therefore, several indirect approaches have been taken to assess soil as a habitat of thermophilic fungi. Eggins et al. (1972) used a soil immersion tube (Figure 10.3) for discriminating the mycelial form from the dormant spores. A cellulose paper strip (a source of carbon) was enclosed inside a screen of glass fiber and placed in soil so that only the active mycelium could penetrate through the screen and colonize the substrate. The tubes were removed from the soil at intervals and the paper strips plated on cellulose agar to determine if these were invaded by mycelium from the soil. By this method the thermophilic fungi Humicola grisea and Sporotrichum thermophile were detected in the sun-heated soil. It would appear that this device had the potential of picking out active mycelium but Tansey and Jack (1976) pointed out that an incorrect impression of growth of fungi can occur if spores were passively carried onto the test substrate by soil arthropods or by capillary action. Therefore, these authors studied the
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Figure 10.3 Diagram of soil immersion tube. (From Eggins et al. (1972). With permission from Elsevier.)
development of spores of individual species of thermophilic fungi in petri dishes containing pure cultures that were buried in field soil in the US state of Indiana. All species tested germinated and developed sporulating colonies in the buried plates, leading the authors to the view that the extent and duration of elevated temperature reached in the sun-heated soil are sufficient for thermophilic fungi to grow therein. However, this extrapolation from pure cultures in nutrient media in buried petri dishes is also doubtful since the propagules of thermophilic fungi co-exist with microbes, mesophilic fungi and microfauna that are potential competitors.
Rajasekaran and Maheshwari (1993) attempted to forecast the potential of thermophilic fungi to grow in soil based on competitive growth in mixed cultures under a fluctuating temperature regime. Incubation of soil plates in a programmed incubator at temperature regimes from 24 to 48°C, 32 to 48°C and 36 to 48°C (Figure 10.4) yielded mesophilic fungi or a thermotolerant fungus, Aspergillus fumigatus. At a temperature regime from 36 to 48°C, the predominant fungi that developed in soil plates were still mesophilic types with occasional colonies of the thermophilic fungus Humicola grisea var. thermoidea. Thermophilic fungi developed only when the incubation temperature fluctuated to a small extent between 40 and 48°C, about the summed average optimum temperature (46°C) of common thermophilic fungi. The results suggested that high temperatures are necessary for thermophilic fungi to compete with the numerically larger mesophilic population in soil. Several workers have isolated thermophilic fungi from desert soils that heats up to 50°C or more with the implied conclusion that desert soil is a natural habitat of thermophilic fungi. However, since liquid water is essential for growth, it is unlikely that the presence of spores of thermophilic fungi in dry soils is a consequence of their growth in situ.
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